The present invention relates to a structure and method for enhanced cooling of generator rotors.
The power output rating of dynamoelectric machines, such as large turbo-generators, is often limited by the ability to provide additional current through the rotor field winding because of temperature limitations imposed on the electrical conductor insulation. Therefore, effective cooling of the rotor winding contributes directly to the output capability of the machine. This is especially true of the rotor end region, where direct, forced cooling is difficult and expensive due to the typical construction of these machines. As prevailing market trends require higher efficiency and higher reliability in lower cost, higher-power density generators, cooling the rotor end region becomes a limiting factor.
Turbo-generator rotors typically consist of concentric rectangular coils, which are made of copper turns radially stacked in slots in a rotor body. The end portions of the coils (commonly referred to as endwindings), which are beyond the support of the main rotor body, are typically supported against rotational forces by a retaining ring (see FIG. 1). Blocks are placed intermittently between the concentric coil endwindings to maintain relative positions and to add mechanical stability (see FIG. 2). The blocks are classified as space, spacer, and wedge blocks, depending on their location.
As noted above, efficient cooling is a prime requirement for a good ventilation design. In a typical generator rotor, cold flow from the fan enters into the passage below the centering ring and on to the endwinding region. This flow further divides into groove flow (in grooved endwindings) and subslot flow. The wedge blocks which are in the vicinity of the rotor body are referred to as body wedge blocks. The cooling flow that passes beneath and around these blocks enters into the subslot ducts which leads the cooling fluid further downstream into the rotor body. While the rotor is rotating at high speed, flow beneath the coils in the endwinding region travels at a high relative tangential velocity. When flow reaches the vicinity of the subslot ducts, due to the high relative tangential component of velocity, it enters the subslot duct at an angle. This misalignment between the flow and the subslot induces losses at the subslot entrance. Further, when the cooling flow enters into the small subslot ducts from the relatively big endwinding duct, the flow contracts suddenly. This is due to the fact that the subslot ducts are defined perpendicular to the large endwinding duct. The contraction of the flow from the large endwinding duct to the small subslot duct is sudden. The sudden contraction induces additional pressure losses.